Most electrical systems and devices are occasionally subject to excessive transient voltages being applied and such voltage surges, e.g. due to lightning strikes, may result in very costly damages and subsequent service interruption. Means for protecting the electrical systems and devices are therefore needed.
Voltage dependent resistors, varistors, are frequently used for protecting electrical devices from voltage surges. The varistor is also referred to as non-linear resistor as it has nonlinear current-voltage characteristics. If an applied voltage is less than a certain voltage, the varistor is essentially an insulator. If the voltage applied is above the certain voltage, the switching voltage, the varistor resistance drops and allows an increased current to flow through it. The varistor is connected in parallel to the device to be protected and arranged to, when triggered by an overvoltage, shunt the current created by the high voltage away from the device.
Further, various kinds of circuit breakers are available for interrupting fault currents. However, it is difficult to design circuit breakers for DC currents due to the high currents without any zero current crossings at which to break the current.
One known high voltage direct current (HVDC) circuit breaker based on insulated-gate bipolar transistors (IGBT) technology comprises a number of stack-mounted IGBT units. Each unit comprises several IGBT modules connected in parallel, each module in turn comprising a number of paralleled semiconductor chips which normally carry only a fraction of a line voltage. If one such semiconductor chip (IGBT chip) is destroyed by overvoltage, it is short-circuited by an internal device which has to withstand the full line current. The short-circuiting device has a limited current carrying capability and the complete IGBT unit has to be replaced within short.
For DC breaker applications, having a large continuous DC current without any zero current crossings which would enhance the transitions from one failed IGBT chip to another, the failed IGBT chip cannot stay in the short circuit mode for very long. The DC breaker thus has to be tripped and the failed device has to be replaced.
There is thus need for improvements in this regard.